The discovery of cyclosporine has made successful organ transplantation possible and has provided insight into a new class of agents to induce selective immunotolerance. A number of hypotheses have been advanced regarding the mechanism of action of cyclosporine, including that it acts as a selective inhibitor of transcription, and as a calmodulin, cyclophilin or prolactin antagonist. It is now recognized that cyclosporine may also produce nephrotoxicity, hepatotoxicity, and central nervous system toxicity. The mechanism by which cyclosporine produces toxicity is unknown. We have demonstrated that cyclosporine does not produce toxicity by acting as a transcription inhibitor when administered in vivo or in vitro. Adding cyclosporine to microsomes in vitro produces no effect on translation; however, when cyclosporine is injected for 10 days and microsomes are isolated, there is a profound inhibition of translation. The inhibitory factor is observed to reside in both the particulate microsomal fraction and cellular supernatant of tissues of cyclosporine-treated animals. We have hypothesized that this factor may be cyclosporine, a cyclosporine metabolite, or an induced or altered protein or factor involved in the regulation of translation. This proposal seeks to characterize the capacity of cyclosporine to inhibit translation. These studies include the examination of translation inhibition in a wide variety of rat tissues, dose- response curves and onset-offset curves for translation inhibition, and examination of inhibition following chronic low-dose cyclosporine. Concomitant with these studies, we will characterize cyclosporine-induced functional changes in the kidney and then correlate them with translation inhibition. Renal function tests include renal proximal tubule reabsorption, tubular transport, creatinine, PAH and inulin clearance, enzymuria, BUN, RBF and GRF. Renal histopathology will also be examined. We will then isolate, purify and characterize the translation inhibitory factor. It is anticipated that these experiments will not only define the mechanism of cyclosporine toxicity, but they may also provide a new hypothesis of the mechanism of cyclosporine action. These experiments are expected to provide insight into means of reducing or circumventing cyclosporine-induced toxicity.